A conventional television tube comprises an almost plane faceplate or screen of rectangular shape. The screen is furnished on its internal face with a mosaic of patches of phosphors or pixels which, excited by an electron beam, emit light which may be blue, green or red, depending on the phosphor excited.
An electron gun sealed in the envelope of the tube is directed towards the centre of the screen and makes it possible to emit the electron beam towards the various points of the screen through a perforated mask (or shadow mask). The electron gun makes it possible to focus the electron beams onto the internal face of the screen carrying the phosphors and to make them converge there.
A deviating system placed around or on either side of the tube makes it possible to act on the direction of the electron beam so as to deviate its trajectory. Continual action of the deviating system thus allows horizontal and vertical scanning of the screen so as to scan the entire mosaic of phosphors.
Without deviation of the electron beam and with symmetric electrodes of the gun that create symmetric electric fields in the gun, the electron beam reaches the centre of the screen.
FIGS. 1a and 1b represent an example of an electron gun to which the invention is applied.
This electron gun comprises a cathode K emitting electrons by thermoemission. An electrode G1 in cooperation with the electrode G2 initializes the formation of an electron beam along the axis Z from the electrons emitted by the cathode.
The electrode G2 focuses the beam thus constituted to a focusing point, called the “crossover”. The size of this focusing point is as point-like as possible. By way of example, the electrode G1 is at a static potential lying between earth and 100 volts. The electrode G2 is at a potential lying between 300 volts and 1200 volts.
The electrode G3 raised, according to this example, to a potential of between 6000 and 9000 volts helps to accelerate the electrons.
The electrode G4 raised to a potential substantially equivalent to that of the electrode G2 constitutes with the electrode G3 and the part of the electrode G5 facing G4 a prefocusing electron lens for the electron beam as is represented in FIG. 1b. 
The electrodes G5, G6 and G7 constitute quadrupolar lenses and will induce a quadrupolar effect on the beam in such a way as to exert a compressive load on the electron beam in the vertical plane and a distortion in the horizontal plane. As described previously, the deformations of the beam are bigger at the periphery of the screen and in particular at the corners of the screen. They increase continuously from the centre of the screen to the periphery. The set of electrodes or quadrupole G5, G6, G7 must therefore carry out a precorrection as a function of the deviation of the beam. This correction must be carried out continuously in synchronism with the screen scanning system. The makeup of the quadrupole created by G5, G6, G7 and the control of the electrodes will be described later.
The device G7-G8 achieves a quadrupolar effect which tends to exert on the electron beam a compressive load in the horizontal plane and a distortion in the vertical plane as was described in relation.
The electrode G9 is the electrode which together with G8 constitutes the principal exit lens.
In a three-colour tube of “in-line” type, the electron gun makes it possible to handle three electron beams (red, green and blue) disposed in one and the same plane. For this purpose, the electron gun possesses electrodes furnished with three holes disposed in line for handling three electron beams.
The invention relates to the main focusing lens of an electron gun of “in-line” type used in three-colour cathode-ray tubes (CRT).
An electron gun is characterized by the following properties:                focus voltage Vf (FIG. 5) and anode voltage (FIG. 5) making it possible respectively to focus and to accelerate the electron beams to the screen. In the case of a Dynamic Focus Modulation gun (DFM gun), the focusing voltage is dynamic and called Vd (FIG. 5),        a “bias” which is defined as being the difference between Vd and Vf (bias=Vd−Vf) at the centre of the screen.        a “delta focus” which is defined as being the difference between the focusing voltage (Vdext) allowing the outer electron beams (red and blue beams for example) to focus at a point on the screen (at the centre for example) and the focusing voltage (Vdint) allowing the central beam (green beam for example) to focus at the same point. As a general rule, it is essential for the “delta focus=Vdext−Vdint” to be zero.        the convergence of the three electron beams (red, green and blue) at the centre of the screen is defined as the manner of impact on the screen of the outer beams (red and blue beams for example) with respect to the central beam on the screen (green beam for example).        
Generally, the “delta focus” is corrected by modifying the diameters of horizontal holes (φH1, φHint=2Rhint and φHext=2RHext in FIG. 3a). In certain configurations, it is not possible to correct the entire delta focus by modifying these diameters through the use of hardware. It has therefore been necessary to find a new parameter for adjusting the “delta focus”.
The invention makes it possible to adjust the “delta focus” by modifying the shape of the edges of the electrodes of the main focusing lens.